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The generation of free charges in efficient organic solar cells

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Organic semiconductors are substantially cheaper than their conventional competitors, and organic LEDs have already been commercialised. It is hoped that the development of efficient organic solar cells might dramatically reduce the cost of renewable solar technologies. However most devices manufactured to date perform very poorly. The reason is simple, organic materials have low dielectric constants (ε ~ 3-4). When light is absorbed it generates a strongly bound electron-hole pair, which must be separated into free charges before a current can be extracted from the device. In most devices, the charge separation timescale is longer than the electron-hole recombination timescale, and very little energy can be extracted. In recent years, a small number of efficient devices have emerged, in which electrons and holes can be separated with near-unity efficiency. The origin of the improved device performance in efficient devices is poorly understood, particularly as researchers often assume that the motion of charges within organic devices can be described by “hopping” models between localised electronic states. In a combination of experimental and theoretical work (Gélinas et al., Science 343, 512), we conclusively demonstrate that the electron and hole can separate by several nm within just 100 fs; and we show that these results are explained by a simple minimal model, which requires the emergence of a set of delocalised states. The emergence of these states arises from superior crystallinity within efficient devices.

Free charges diffusing through the device may re-encounter each other, and run the risk of delayed “non-geminate” recombination. I will extend the model developed above to explore this process, and demonstrate that these same delocalised states are also able to suppress this secondary loss mechanism (Smith and Chin, PCCP 16, 20305), consistent with a range of recent experimental observations, and providing a complete picture of the charge generation processes within efficient devices.

This talk is part of the Electronic Structure Discussion Group series.

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